The broad objective of our work is to understand the neural regulation of the heart under a variety of natural and experimental conditions. The specific objective of the present proposal is to investigate the functional significance of the cardiac and respiratory periodicities observed in both divisions of the autonomic nervous system. With respect to the vagal control of the heart, attention will be directed principally toward extending our knowledge of the phase-related effects of vagal activity. Experiments will be conducted to distinguish between the effects of interstimulus interval and of phase on the following aspects of cardiac function: heart rate, atrioventricular conduction and the automaticity of certain ectopic pacemaking centers. The phase-related effects of vagal stimulation will be studied over a range of levels of cardiac sympathetic activity and over a range of doses of certain rapidly acting digitalis preparations. The influence of the interval between individual pulses in a vagal stimulus burst will be determined on various cardiac responses, such as heart rate, A-V conduction time, and myocardial contractility.